1. Field of the Invention
The present invention relates to an improvement of a silent discharge gas laser having high output power.
2. Description of the Prior Arts
Referring to FIG. 1, a transversal excitation CO.sub.2 laser, the conventional continuous oscillation type silent discharge gas laser is illustrated.
In FIG. 1, the reference numeral (1) designates a grounded electrode; (2) designates high voltage metal electrode which is arranged to the grounded electrode (1) to form a pair of discharge electrodes. The discharge surface of the high voltage metal electrode is covered by a dielectric layer. The reference numeral (4) designates a discharge gap; (5) designates a transformer; (6) designates a high frequency power source; (7) designates a total reflector; (8) designates a partial reflector; (9) designates a coolant water recycling pump; (10) designates a cooler; and (11) designates an ion-exchange type water deionizer.
The operation of the silent discharge gas laser will be illustrated.
When high frequency high voltage power is applied to the high voltage metal electrode (2) by the high frequency power source (6) and the transformer (5), a stable discharge as a silent discharge is formed across the discharge gap (4). The silent discharge is an AC discharge formed through the dielectric layer (3) between the high voltage metal electrode (2) and the grounded electrode (1), whereby a stable unbalanced discharge having high electron temperature can be formed without increasing the molecule temperature and without causing any arcing. The description of light induced radiation step caused by molecules excited in the discharge gas (4) is not discussed in detail. A laser oscillation occurs in the resonator comprising the total reflector (7) and the partial reflector (8) due to the silent discharge in the discharge gap (4) whereby the laser radiation l is emitted from reflector (8) at the output side.
In this case, the high voltage metal electrode (2) is cooled by coolant water having a low electric conductivity. The coolant water is recycled through the passage between the high voltage electrode and the pump including (9) cooler, (10) water deionizer and (11) high voltage electrode. In the water recycling, a deionization of the coolant water (decreasing electric conductivity) is attained so as to prevent the electric leakage from the high voltage metal electrode (2) in the ion-exchange type water deionizer (11).
The gas (not shown) in the discharge gap (4) is passed between the grounded electrode (1) and the high voltage metal electrode (2) perpendicular to the laser radiation and the discharge direction, but parallel to the surfaces of the electrodes.
Referring to FIGS. 1(II)a and 1(II)b, the silent discharge will be illustrated.
As described above, the silent discharge is an AC discharge which occurs through the dielectric layer (3). The voltage V.sub.gap in the discharge gap (4) is increased depending upon the voltage of the power source. When the potential difference in the discharge gap reaches the discharge starting voltage V.sub.dis, a pulse discharge which exists for a short time is formed in dispersion in the discharge gap. When the discharge starts, the charge caused by the discharge current is accumulated on the surface of the dielectric layer (3). As a result, the potential difference in the discharge gap (4) is decreased to stop the pulse discharge. When the voltage of the power source is increased again to cause the voltage in the discharge gap (4) to reach the discharge starting voltage, the discharge occurs again. The discharge is repeated for several to several tens times in a half cycle of the AC power source. In FIGS. 1(II)a and 1(II)b, the reference I designates the average value of the current. In the next half cycle, the discharge in reverse polarity is repeated in the similar manner. The power W is given by the equation: EQU W=V.times.I as FIG. 1(II)b.
FIGS. 2(a), 2(b), 2(c) show variations in the voltage of the silent discharge, power and the output of the laser in the case of the silent discharge gas laser which operates at a frequency f=5 KH.sub.z.
The laser medium gas comprises CO.sub.2, N.sub.2 and He at ratios of partial pressures of 5:60:35 under a total pressure of 70 Torr. The electrode gap is 20 mm; and the electrode length is 1 m. On the surface of the high voltage metal electrode (2), the dielectric layer (3) made of a borosilicate glass is covered by sintering.
FIG. 2(a) shows a waveform of the applied voltage V (peak value of 10 KV).
FIG. 2(b) shows a waveform of the making power (W); and
FIG. 2(c) shows a waveform of the output of the laser.
The discharge power is intermittently charged as shown in FIG. 2(b), whereas the output of the laser is substantially constant in time as shown in FIG. 2(c). However a ripple component having a frequency of 2 f and an intensity of about 10% is superposed because the power is intermittently charged as shown in FIG. 2(b).
When a substrate having a large heat conductivity is worked by the output of the laser (as metal works), certain heat energy externally escapes through the substrate. Thus, when an average output is equal, the output of the laser having higher peak value is advantageous over the output of the laser being substantially constant in time as shown in FIG. 2(c).
From the above-mentioned viewpoints, the oscillation output (P) can be studied under various conditions to find that the time constant in the increasing or decreasing of the ripple is dependent upon the composition and the pressure of the gas. The intensity of the ripple is varied depending upon the composition and the pressure of the gas. The above-mentioned time constant is equal to the time constant .tau. in the generation or the extinction of the output of the laser in a sudden application or a sudden stop of the high frequency voltage. (the time for imparting an intensity of 1/e times). The results of the measurements are shown in Table 1.
The time constant is decreased depending upon the increase of the pressure of a gas and the decrease of the ratio of the partial pressure of N.sub.2.
TABLE 1 ______________________________________ Time constant .tau.(m sec.) in variation of output of laser; Kind of gas Gas pressure (Torr) CO.sub.2 --N.sub.2 --He 70 150 200 ______________________________________ 5:60:35 0.3 .+-. 0.1 0.15 .+-. 0.05 0.1 .+-. 0.03 5:20:75 0.1 .+-. 0.03 0.05 .+-. 0.02 0.03 .+-. 0.01 ______________________________________